Methods for implementing a Rankine cycle within a system to recover thermal energy from an engine are well known. Although these systems were initially developed to produce steam that could be used to drive a steam turbine, the basic principles of the Rankine cycle have since been extended to lower temperature applications by the use of volatile organic chemicals as propellants with the system. Such organic Rankine cycles (ORCs) are typically used within thermal energy recovery systems or geothermal applications, in which heat is converted into secondary mechanical energy that can be used to generate electrical energy. As such, these systems have become particularly useful in heat recovery and power generation—collecting heat from turbine exhaust gas, combustion processes, geothermal sources, solar heat collectors, and thermal energy from other industrial sources. Organic Rankine cycles are generally most useful within temperature ranges from 158 to 752 degrees F., and are most often used to produce power between 400 kW and 5000 kW of power.
Generally, a Rankine-based heat recovery system includes a propellant pump for driving propellant through the system, an evaporator for evaporating propellant that has become heated by collection of waste heat, a turbine through which evaporated propellant is expanded to create power or perform work, and a condenser for cooling the propellant back to liquid state so it may be pumped to collect heat again and repeat the cycle. The basic Rankine cycle has been adapted for collection of heat from various sources, with conversion of the heat energy to other energy outputs.
For example, U.S. Pat. No. 5,440,882 describes a method for using geothermal energy to drive a modified ORC based system that uses an ammonia and water mixture as the propellant. The evaporated working fluid is used to operate a second turbine, generating additional power. Heat is conserved within the Rankine cycle portion of the system through the use of a recuperator heat exchanger at the working fluid condensation stage.
U.S. Pat. No. 6,986,251 describes a Rankine cycle system for extracting waste heat from several sources in a reciprocating engine system. A primary propellant pump drives the Rankine cycle with assistance from the auxiliary booster pump, to limit pump speeds and avoid cavitation. When the Rankine cycle is inactive (e.g. due to reciprocating engine failure or maintenance), the auxiliary pump operates alone, circulating propellant until the propellant and system components have cooled sufficiently for complete shut down. Diversions are present to prevent circulation of propellant through the evaporator and through the turbine during this cooling cycle.
U.S. Pat. No. 4,228,657 describes the use of a screw expander within a Rankine cycle system. The screw expander is used to expand a thermodynamic fluid, and waste heat is further extracted from the expander in order to improve system efficiency. A geothermal well supplies pressurized hot water or brine as the heat source.
When using organic propellants within a Rankine cycle, care must be taken to avoid exposure of the propellants to flame. Although specialized organic propellants having high flash temperatures (for example Genetron® R-245fa, which is 1,1,1,3,3-pentafluoropropane) have been developed, the danger of combustibility still exists, as engine exhaust may reach temperatures up to 1200 degrees F. A leak in an exhaust heat exchanger could therefore be disastrous. Further, the purchase of proprietary propellants adds a significant start-up cost to these systems.
A common problem particularly relevant to recovery of thermal energy is that when using air-cooled condensers, ambient air temperatures significantly impact the system efficiency and total power available.